Urban hydrology Brent King Senior science coordinator GWRC Reuben - - PowerPoint PPT Presentation

Urban hydrology

Brent King Senior science coordinator GWRC Reuben Ferguson Water resources scientist Morphum Environmental Sue Ira Director Koru Environmental

Aim of the session

• Identify the Committee’s preferences

for managing urban development for its impact on stream flows

Some info to help

• Why manage the effects of runoff?
• What does different infill and greenfield

development practice get you?

– How and why does hydrology differ – How do the effects on stream health differ – How and why do costs differ

Why manage the runoff effects?

• One of many factors that affect stream

ecology

– Water quality (eg, toxicants, temperature) – Flow (eg, low flows, total runoff, disturbances) – Habitat (eg, bank stability, stream bed, shade, passage)

Why manage the runoff effects?

• People need places to live
• More hard surfaces

– more runoff – faster runoff

• Cost to develop and live there
• Resilience
• Climate change
• Amenity of the built environment

Your scenarios

• Package of interventions covering

many factors that affect stream health

• Today’s focus is on urban runoff
• Improved

– Slow the water down – Limited removal of runoff reaching the stream

• Water sensitive

– Reduce the area of hard surfaces – Slow the water down – Store and remove runoff from reaching the stream

Case studies

• Relatively small catchments with lots
• f urban development in the scenarios
• Magnify the effects of urban

development and changes in practice within each development type

• Results likely to be less dramatic in

mixed land use catchments

Results to look for

• Annual average runoff

– changes indicate a likely shift in the stream ecology towards less diverse macroinvertebrate communities with fewer sensitive species

• Frequency of bed disturbing flows

– too much disturbance can mean only the tolerant species (typically the small and rapidly species colonising species) survive while sensitive species are lost

• Life cycle costs per household

– Difference in costs to install and maintain the scenario bundles of stormwater mitigation measures – Split of private and public borne costs

URBAN HYDROLOGY MODELLING

TE AWARUA-O-PORIRUA WHAITUA

Case study catchments

Greenfield Infill

• Converts rainfall into stormwater runoff
• Compare changes in runoff across development scenarios
• Effect of landuse changes
• Effect of stormwater management devices

– Rainwater tanks (reuse of captured water) – Raingardens in streets – Wetlands (reuse of treated water) – Permeable paving

• Report results
• Annual volume of runoff
• Number of bed-disturbing flows per year

Hydrology model

• Existing
• BAU
• Higher dwelling density, greater imperviousness
• No mitigation of runoff
• Improved
• Same density and imperviousness as BAU
• Some mitigation of runoff using devices = stormwater detention
• WSUD
• Same density but reduced imperviousness
• Extensive use of runoff mitigation devices = stormwater retention

Model scenarios

Model structure (Infill catchment)

Results – Runoff volume

Greenfield Infill

Greenfield

BAU to WSUD: 53% Improved: 10% WSUD: 43%

Infill

BAU to WSUD: 42% Improved: 6% WSUD: 37%

% volu

• lume reduction

Results – Bed disturbing flows

Greenfield Infill

• Improved scenario
• Reduces frequency of bed-disturbing flows
• Has small effect on total runoff volume
• WSUD scenario
• Reduces bed-disturbing flows and total runoff volume
• Greatest benefit through stormwater retention
• Re-use collected rainwater within houses (constant daily use)
• Infiltration of stormwater (less viable)
• WSUD approach has wider benefits than just hydrology
• Water quality improvement
• Amenity enhancement
• Reduced demand on mains water supply (rain tanks)
• Resilience, e.g. household water supply following earthquake

Key messages

Results – Cumulative frequency distribution

At t 95% Existing: 6 L/s BAU: 20 L/s Improved: 12 L/s WSUD: 5 L/s

THE COST AGGREGATION MODEL

PORIRUA WHAITUA

Synopsis….

• What is life cycle costing and how can we use it?
• Information on the Porirua Whaitua LCC models (assumptions)
• LCC results for our two case study catchments

What is is lif life cycle costing (L (LCC)?

Definition:

“…..the process of assessing the cost of a

product over its life cycle or a portion thereof…..”

Ref: Australian/New Zealand Standard 4536:1999 Treasury New Zealand

Phases in the life cycle of a stormwater practice and potentially associated costs (Taylor, 2003) $ Cost

How th the stormwater LCC model l works

• Builds on existing LCC Work
• Based on generating a total LCC which includes analysis of TAC and

maintenance costs over a 50 year analysis period (base date of 2017)

• Relates to:
• best practice design of the mitigations
• impervious area treated
• treatment performance

Understandin ing how to use LCCs

• Allows comparison of costs of one or more devices against another
• Balances performance (benefits) against cost
• Use ranges to express uncertainty due to data gaps or large variation

in costs (focus on ranges rather than absolutes)

• Look for patterns and relative differences between scenario results
• Today’s results are indicative based on the interventions and

dwellings in our two case studies only.

Cost Result lts – urban costs

$260,000 $850,000 $144,000 $480,000 $290,000 $670,000 $201,000 $495,000 $- $100,000 $200,000 $300,000 $400,000 $500,000 $600,000 $700,000 $800,000 $900,000 Low High Low High CAMBORNE GREENFIELDS CASE STUDY KENEPERU INFILL CASE STUDY

TOTAL INDICATIVE ESTIMATE LCC $/ YEAR

IMPROVED SCENARIO WSUD SCENARIO

Cost Result lts – urban costs

$360 $1,100 $40 $140 $400 $900 $60 $150 $- $200 $400 $600 $800 $1,000 $1,200 Low High Low High CAMBORNE GREENFIELDS CASE STUDY KENEPERU INFILL CASE STUDY

TOTAL INDICATIVE ESTIMATE LCC $/YEAR/DWELLING

IMPROVED SCENARIO WSUD SCENARIO

$360 $1,100 $40 $140 $400 $900 $60 $150 $- $200 $400 $600 $800 $1,000 $1,200 Low High Low High CAMBORNE GREENFIELDS CASE STUDY KENEPERU INFILL CASE STUDY

TOTAL INDICATIVE ESTIMATE LCC $/YEAR/DWELLING

IMPROVED SCENARIO WSUD SCENARIO

Cost Result lts – whic ich costs to use?

Cost Result lts – public lic / / priv ivate spli lit (u (urban)

81% 19%

CAMBORNE GREENFIELD CASE STUDY - IMPROVED SCENARIO Proportion of total LCC as public or private

Public portion of total LCC$/yr Private portion of total LCC$/yr

59% 41%

CAMBORNE GREENFIELD CASE STUDY - WSUD SCENARIO Proportion of total LCC as public or private

Public portion of total LCC$/yr Private portion of total LCC$/yr

Cost Result lts – public lic / / priv ivate spli lit (u (urban)

81% 19%

KENEPERU INFILL CASE STUDY - IMPROVED SCENARIO Proportion of total LCC as public or private

Public portion of total LCC$/yr Private portion of total LCC$/yr

57% 43%

KENEPERU INFILL CASE STUDY - WSUD SCENARIO Proportion of total LCC as public or private

Public portion of total LCC$/yr Private portion of total LCC$/yr

Take home messages…...

• Costs are indicative estimates of LCCs – relative difference between

scenarios

• The difference in costs between the ‘improved’ and ‘water sensitive’

scenarios are relatively small

• Wetlands are a major driver of the large ranges in cost estimates,

particularly for the ‘improved scenario’

Take home messages…...

• Keneperu – use high-end of cost range of estimate (infill)
• Taupo at Camborne – use low-end of cost range estimate (greenfield)
• “Improved’ scenario models a high share of public costs from

catchment scale methods to slow water down

• “Water sensitive” scenario models higher shares of privately borne

costs from the higher use of lot scale retention and in home reuse